GDB + softdevice ?

[ypearson]

Can you use GDB for your app code if soft device was merged in the final hex?

[RK]

In general you can't single-step through softdevice calls, or break while the softdevice is running, with any debugger because the softdevice has timing requirements which single stepping breaks.

Here's a recent blog about monitor mode debugging which attempts to work around this

devzone.nordicsemi.com/.../

but it's an advanced technique. This all has nothing to do with GDB.